1. Field of the Invention
This invention relates to a desiccant effective for removal of water from HFC-32 and HFC-152a, which are used as refrigerants for refrigerators and air-conditioners.
2. Description of the Prior Art
Refrigerators, freezers, air conditioners, and other such machines require a refrigerant to operate.
If the refrigerant circulated inside the machine contains water, it causes various problems.
First, when the refrigerant is adiabatically expanded in the freezing cycle, the water entrained thereby freezes into ice because of the sudden decrease in temperature and the ice obstructs the circulation of the refrigerant.
Second, the water reacts with the refrigerant or compressor oil and produces acids such as hydrofluoric acid and hydrochloric acid and these acids corrode the metals used in the system.
Third, the compressor oil is degraded by the water or by the acids produced because of the presence of the water as described above and the resulting sludge blocks the circulation.
Fourth, the electrochemical reaction causes copper plating, namely plating of the inner wall of the copper piping with iron ions.
The water must therefore be removed from the refrigerant. The freezers, air conditioners, etc. currently in use chlorofluorocarbon type refrigerants, typically CFC-12 and HCFC-22. Synthetic zeolites have been used as practical desiccants for these refrigerants.
Synthetic zeolites are available in various types and the best one for the particular type of refrigerant is selected.
The general standard for the selection of a synthetic zeolite as a desiccant will now be explained. The molecules of synthetic zeolites have micropores on the nanometer order. These micropores adsorb molecules with diameters smaller than the effective diameter of the micropore but do not adsorb molecules with diameters larger than the effective diameter.
The selection of a desiccant will now be described with respect to CFC-12 as a concrete example of the refrigerant.
The diameter of a molecule of CFC-12 is found by calculation to be 0.44 nm and that of a molecule of water to be 0.21 nm.
Thus, a 4A type zeolite, which has an effective diameter of 0.4 nm, is used as the desiccant for CFC-12. Since the diameter of a molecule of HFC-12 is larger than the effective diameter of the 4A type zeolite, the refrigerant CFC-12 is not adsorbed by the desiccant. Since the diameter of a molecule of water is smaller than the effective diameter of the 4A type zeolite, water is adsorbed by the desiccant. Thus, the 4A type zeolite removes the water contained in the refrigerant HFC-12 by adsorption.
The refrigerant HCFC-22 has a molecular diameter of 0.38 nm. The 4A type zeolite is therefore not effective as a desiccant for this refrigerant. In contrast, the 3A type zeolite, which is derived from the 4A type zeolite by substitution of potassium ions for part of the sodium ions in the 4A type zeolite and has an effective microporous diameter of 0.3 nm, is effective as a desiccant for the refrigerant HCFC-22.
Use of the compounds CFC-12 and HCFC-22 still widely employed as refrigerants is being progressively limited are destined to and will eventually be totally banned.
It has been ascertained that the compound CFC-12 destroys the earth's ozone layer. As a result, the ozone layer absorbs less of the Sun's ultraviolet rays and allows more of this light to reach the earth's surface, where it is a cause of skin cancer. Under international agreements the use of this compound is to be abolished in 1996. In the light of reports that the compound HCFC-22 bears heavily on global warming, a plan to restrict the use of this compound in the near future is also gradually taking shape.
As substitutes for the compounds CFC-12 and HCFC-22, the fluorocarbons HFC-32, HFC-152a and blended refrigerants containing HFC-32 and/or HFC-152a are expected to find popular acceptance since they contain hydrogen atoms and no chlorine atom and have no possibility of destroying the ozone layer or warming the earth.
Efforts are therefore being focused on the early development of a method for the desiccation of HFC-32 and HFC-152a so as to enable their practical application.
Because the calculated molecular diameters of HFC-32 and HFC-152a are 0.33 nm and 0.39 nm and the molecular diameter of water is 0.21 nm, based on the selection standard discussed above it would appear that 3A type zeolite, which has an effective microporous diameter of 0.3 nm, would be an ideal desiccant for these refrigerants.
It has been ascertained, however, that the refrigerants HFC-32 and HFC-152a are both adsorbed by the 3A type zeolite and experience reactions such as decomposition. From this it is clear that 3A type zeolite is unsuitable as a desiccant.
It might be thought that this problem could be overcome by further decreasing the effective microporous diameter of a desiccant such as, for example, zeolite, by deformation, for example. However, among all of the practical species of zeolite which are inexpensively producible by the ion-exchange method, it is the 3A type zeolite that has the smallest effective microporous diameter. Moreover, when the micropores in the zeolite are deformed as by firing at a high temperature, the zeolite's capacity for adsorbing water is degraded.
Under the circumstances, there is a strong need for developing a desiccant which adsorbs substantially no HFC-32 or HFC-152a but possesses a high capacity for adsorption of water.